FIG. 9 is a view illustrating the configuration of an LD-pumped solid-state laser oscillator. As shown in FIG. 9, the solid-state laser oscillator is constituted by one or more cavities 3, each of which is sandwiched by two mirrors that are a partial reflection mirror 1 and a total reflection mirror 4, so that desired laser beams 5 are taken out from the oscillator. FIG. 10 is a schematic view illustrating the cavity. The cavity 3 is configured so that a solid-state pumping medium 7 is pumped by light irradiated from an LD 6 serving as a pumping light source. FIG. 9 shows the laser oscillator having two cavities. Several tens or hundreds of LDs 6 are arranged to be connected in series so that the solid-state pumping medium 7 is uniformly pumped by an output of the laser oscillator or by an output per LD.
Laser power available from one solid-state pumping medium changes in the volume (shown as a hatched part in FIG. 10 and hereunder referred to a “mode volume”) 21 of the solid-state pumping medium 7, through which laser light being in a resonant condition can pass, and increases when the mode volume 21 increases. The mode volume 21 increases or decreases when the output of the LD 6 increases or decreases. In a case where a pumping distribution in the solid-state pumping medium 7 is uniform, the mode volume uniformly exists uniformly with respect to the central axis 10 of the solid-state pumping medium 7, as illustrated in FIG. 10. Generally, a laser output from the cavity is nearly proportional to an LD output that is approximately proportional to an LD energizing current. Thus, to obtain a desired laser output, generally, techniques of controlling an LD current are used. A solid-state laser oscillator required to obtain a high output employs a configuration in which a plurality of cavities are arranged between a partial reflection mirror and a total reflection mirror to thereby obtain a laser output that is a sum of outputs of the cavities.
Next, a failure of the LD is described below.
Failure modes of the LD 6 are a shortcircuit failure mode and an open failure mode. In a case where a short circuit failure occurs, the LD 6 does not emit light, so that an unpumped part is generated in the solid-state pumping medium 7. Thus, the mode volume 21 decreases. Consequently, a laser beam output from the laser oscillator decreases. A method of correcting the laser beam output by increasing an LD energizing current and more strongly pumping the entire solid-state pumping medium 7 is employed as a method of making up for a reduction in the laser beam output.
On the other hand, in a case where an open failure of the LD 6 occurs, an electric current path for energizing the LD 6 is interrupted, so that all the LDs 6 including the LDs 6 which do not malfunction are turned off. The solid-state pumping medium is not pumped. The laser oscillator stops. Consequently, no laser beams can be outputted. A configuration, in which the LDs 6 are parallel-connected to one another, is cited as a countermeasure against the stoppage of the oscillator. This configuration needs a power supply capable of supplying electric current, the amount of which is n times (“n” designates the number of LDs) that of electric current required in the case of series-connecting the LDs, and peripheral equipment. Therefore, the structure and the cost of this configuration are impractical. Thus, a method of series-connecting the LDs 6 and providing a bypass circuit in parallel to each of the LDs 6 to thereby form an electric current path at an open failure of the LD 6 has been in common use. Because the bypassed LD 6 is turned off, the laser output is lowered. A method of increasing the LD energizing current and more strongly pumping the entire solid-state pumping medium 7, similarly to the case of the shortcircuit failure, is employed as the method of making up for a reduction in the laser output.
Patent Document 1: JP-A-10-284789
Patent Document 2: JP-A-59-103565